Feedforward Control for CCD Circuit

Hi,

I am looking at doing some optimization works for a processing plant. The plant utilizes a 5 stage counter current decantation circuit (5 thickeners in series - underflow travels downstream (1-5), overflow travels upstream (5-1)).

For the each thickener there is a PID loop that controls the speed of underflow pumps to maintain a constant bed mass in that thickener. This works OK for the first thickener in the chain but changes in the underflow density of this thickener causes disturbances downstream and disturbances are amplified throughout the chain. I want to add in some feedforward control to adapt to the changes in underflow density to improve response time to them.

My problem is I don't know physically how to implement this feedforward control in the PLC. I have done some research and from what I can gather I will need some kind of mathematical model of how changes in underflow density will affect the bed mass of the next thickener in the chain. This isn't really practical (or possible??) - is there some easier way I can achieve this? I have also read that it may be possible to use a PID loop with only the derivative term used - but I'm not sure how I would tune this. Does anyone have any experience in implementing feedforward control?

Thanks,

AJ

You might consider the following.
Leave control of the first thickener as is but use predominantly proportional control; the response should not be underdamped.
For subsequent thickeners, control bed mass with ratio control (which is a form of feedforward, in this case). Ideally, you'd want to ratio mass flowrate among the tanks but you may not have that luxury depending on what other measurements are available to you. If you only have pump speed available, and all pumps/thickeners are equally sized, you might try ratioing pump speeds. So, it works like this: the PID measure bedmass as it does now but the PID's output sets the ratio of the it's pump speed to the speed of the pump for the thickener that precedes it. Ideally, that ratio should be about 1. Multiply the ratio by the command signal (setpoint) of the preceding pump to get the command signal of the current pump. If you can imagine 'perfect control', as the mass increases in the first thickener, all thickeners' pumps ramp up together to drive the load disturbance through the system. Use only proportional control for the thickeners (and only a very small amount of integral control if necessary).

Good luck.